Discussion of Prior Art
The design concepts and production techniques used in the manufacture of airfoil shaped fan blades have not been significantly changed for quite some time. Conventional construction of airfoil shaped fan blades has been to form the airfoil blade shape using dies and an appropriate metal alloy of sufficient thickness and strength to meet the stress criteria required to maintain mechanical integrity for a fan operating within its specified limits. If higher specified operating limits are required for a particular fan design, the material thickness of the blades and their supporting rotor assembly must be increased for strength. This also adds to the weight of the assembly, thereby also increasing stresses brought about by centrifugal forces. Destructive centrifugal forces are calculated by dividing the new revolutions per minute by the known revolutions per minute, squaring this dividend, and multiplying this squared product by the known stress, thus giving the new stress level; or more simply shown in the following formula: EQU Formula: New stress=Known stress.times.(New revolutions per minute.div.Known revolutions per minute).sup.2
It can be seen, by one knowledgeable of the principles involved, that increasing blade material thickness to increase operating speed range reaches a point of diminishing returns. To overcome this problem, present practice is to change the blade material to a stronger alloy (an alloy with a higher tensile strength) or adding reinforcing ribs to the inside of the blade, the latter being very labor intensive and both solutions add weight or cost, or both, to the assembly. A subsequent detrimental effect of the prior art is, with this increase in weight of the blades and supporting wheel assembly, designed to give greater operating speeds, the other rotating parts of the fan may have to be strengthed or enlarged (for example, fan shafts, bearings, motors, etc.). The aforesaid adds up to a considerable increase in the cost of the fan assembly.
It is also known that hollow shapes can be strengthened by filling cavity with a structural foam plastic, as shown in Kramer U.S. Pat. No. 3,909,058 issued Sep. 30, 1975. A disadvantage of this process is that a very limited increase in strength is achieved.
It is also known that shapes and airfoil sections can be reinforced by filling them with macrosphere clusters formed from bonded microspheres, as described by Murphy and Phillips U.S. Pat. No. 4,405,543 issued Sep. 20, 1983.
It is further known that a similar method of using macrosphere clusters formed of bonded microspheres to reinforce structural members was shown by Wycech U.S. Pat. No. 4,695,343 issued Sep. 22, 1987.
A disadvantage to both of the aforesaid processes is that the clusters are made up of groups of bonded hollow microspheres, this forming irregular-shaped macrosphere clusters, with the microspheres used in these macrosphere clusters ranging in size from approximately 0.020 mm. to 0.150 mm. Establishing sizing guidelines from the prior art, it has been indicated, generally, that microspheres would be of a size that is less than 0.5 mm., and macrospheres would be of a size that is 0.5 mm. or greater. It can be seen, by one knowledgeable in the art, that these macrosphere clusters are going to range widely in size and shape, which will keep the macrosphere clusters from flowing smoothly, settling, or packing tightly; thus leaving irregular, open air space interstices between the macrosphere clusters. This prevents the final cured and bonded shape from having the highest compressive strength possible for reinforcement purposes.
It is still further known that a plurality of bonded macrospheres may be bonded into a low density, rigid mass, as stated by Douden U.S. Pat. No. 4,657,810 issued Apr. 14, 1987. A disadvantage to this process is that using hollow spheres of random sizes will not allow for the hollow spheres to pack tightly; thus leaving irregular, open air space interstices between the macrospheres. This prevents the final cured and bonded shape from having the highest compressive strength possible for reinforcement purposes; nor does it imply that a matrix material can be inserted into the open space interstices around the macrosphere clusters to form a solid mass; thus giving the highest compressive strength possible for reinforcement purposes.
In all the aforesaid patents, the uses do not include utilizing the bonded spheroid mass as a reinforcement for a fan blade shape that will be subjected to rotation, and the subsequent centrifugal forces imposed by this rotation.